Wildfires had been burning in the Great Smoky Mountains for a few weeks (see previous blog posts) as extreme to exceptional drought persisted over the region. However, on 28 November 2016 weather conditions became conducive to extreme fire behavior — and this allowed the Chimney Tops 2 Fire south of Gatlinburg, Tennessee to race rapidly northward (fire perimeter map), driven by strong southerly winds gusting to at least 30-40 knots (as were recorded in Knoxville KTYS, located about 25 miles northwest of Gatlinburg). Widespread evacuations were necessary, and at least 13 fatalities were reported. 4-km resolution GOES-13 Shortwave Infrared (3.9 µm) images (above) showed the development of a fire “hot spot” (the cluster of pixels at the center of the images exhibiting a black to yellow color enhancement) during the day, before clouds moved overhead to mask the fire hot spot signature. The warmest infrared brightness temperature seen during this time period was 326.8 K (brighter yellow pixels) on the 1700 UTC image.

Even though cloud cover was increasing, a detailed view of the fire hot spot was provided by an AWIPS II image of 375-meter resolution Suomi NPP VIIRS Shortwave Infrared (3.74 µm) data at 1815 UTC on 28 November (below). An AWIPS I version of this image is available here. Due to the cloudiness, no discernible hot spot appeared on the lower-resolution 1815 UTC GOES-13 Shortwave Infrared image.

Suomi NPP VIIRS Shortwave Infrared (3.74 µm) image [click to enlarge]

Props to NWS meteorologist Carl Jones for spotting this somewhat unexpected result: the glow of the fire was evident on the following nighttime Suomi NPP VIIRS Day/Night Band (0.7 µm) image, even though there was a thick layer of clouds over the fire itself:

An AWIPS II image comparison of VIIRS Infrared Window (11.45 µm), Shortwave Infrared (3.74 µm) and Day/Night Band (0.7 µm) data at 0816 UTC on 29 November is shown below. Cloud-top Infrared Window brightness temperatures were in the -40 to -55º C range over the fire region (such air temperatures were foundd within the 9.5-10.5 km altitude range on the Nashville sounding when the cloud band was over central Tennessee at 00 UTC). While no fire hot spot signature was evident on the Shortwave Infrared image (due to masking by the clouds), the very distinct bright glow of the fire (which appeared rather large in size, due to scattering of light by the water and ice particles present in the various cloud layers) was seen on the Day/Night Band image. AWIPS I versions of these images are available here.

As a follow-up to the previous Otto blog post, GOES-13 Infrared Window (10.7 um) images (above) showed Otto around the time that it became the latest hurricane on record to form in the Caribbean Sea on 22 November 2016 (NHC advisory).

A comparison of GOES-13 Visible (0.63 um) and Infrared Window (10.7 um) images (below) revealed multiple convective bursts during the day, some of which exhibited IR brightness temperatures of -80º C and colder (violet enhancement). Because of Otto’s central dense overcast, no eye was apparent in the GOES-13 imagery; even on a DMSP-16 SSMIS Microwave (85 GHz) image at 2049 UTC the eyewall was not fully closed.

As Otto slowly approached the coast of southern Nicaragua on 24 November, it rapidly intensified (SATCON plot) to a Category 2 hurricane. GOES-13 Infrared Window (10.7 µm) images (above; also available as a 36 Mbyteanimated GIF) and Visible (0.63 µm) images (below; also available as a 18 Mbyteanimated GIF) showed the development of an eye just offshore, which rapidly filled as the storm moved inland after 17 UTC on 24 November and began to interact with the terrain. After crossing Nicaragua and Costa Rica, an eye was once again discernible around 02 UTC on 15 November (as Otto emerged over the Pacific Ocean).

Before the formation of an eye, a Suomi NPP VIIRS Infrared Window (11.45 µm) image at 0639 UTC (below; courtesy of William Straka, SSEC) showed the presence of cloud-top gravity waves propagating westward along the Nicaragua/Costa Rica border; these waves were likely a response to deep convective bursts offshore near the center of Otto.

Suomi NPP VIIRS Infrared Window (11.45 µm) image [click to enlarge]

A comparison of DMSP-17 SSMIS Microwave (85 GHz) and GOES-13 Infrared Window (10.7 µm) images around 1115 UTC on 24 November (below) revealed a much larger (albeit not completely closed) eye signature using the microwave data.

Otto became the southernmost landfalling hurricane on record for Central America. It was also the strongest hurricane on record for so late in the season within the Atlantic basin.

DMSP-18 SSMIS Microwave (85 GHz) image [click to enlarge]

A DMSP-18 SSMIS Microwave (85 GHz) image at 0043 UTC on 25 November (above) showed that the eye of Otto was still well-defined as it began to move into northern Costa Rica (making this the first hurricane or tropical storm on record for that country). The eye structure could be tracked on MIMIC-TC imagery (below) as it moved inland from the Atlantic Ocean, across far southern Nicaragua and far northern Costa Rica, and eventually emerged over the Pacific Ocean after about 03 UTC on 25 November.

As Tropical Storm Otto was weakening during its west-southwestward motion over Pacific Ocean waters with low Ocean Heat Content, nighttime images of Suomi NPP VIIRS Infrared Window (11.45 µm) and Day/Night Band (0.7 µm) data at 0744 UTC on 26 November (above; courtesy of William Straka, SSEC) displayed shorter-wavelength cloud-top gravity waves on the Infrared image and longer-wavelength mesospheric airglow waves (reference) on the Day/Night Band image (all of which were propagating west-southwestward away from the deep convective cluster near the center of Otto). Bright lightning streaks were also seen on the Day/Night Band image.

GOES-13 Visible (0.63 µm) images (above) revealed a series of gravity waves propagating southeastward across the Gulf of Mexico on 22 November 2016. These waves appeared to be moving through or along the tops of the marine boundary layer stratocumulus cloud field; since the hourly winds from surface/buoy/ship reports were generally from the southeast/east/northeast, these gravity waves were not likely surface-based.

Also of note was an apparent land breeze boundary that moved west-northwestward away from the Yucatan Peninsula of Mexico during the day.

Gravity waves like those observed here are usually ducted within a strong air temperature inversion — so Suomi NPP NUCAPS (NOAA-Unique CrIS/ATMS Processing System) soundings around 1828 UTC (above) were examined for evidence of such an inversion. Training material for NUCAPS Soundings in AWIPS is available here.

One of the NUCAPS vertical profiles of temperature and moisture is shown below, for a point over the southern Gulf of Mexico (designated by the cyan rectangle, approximately 50 miles northwest of the coast of the Yucatan Peninsula) — several of the waves had passed through this location prior to the image time. A well-defined temperature inversion did indeed exist aloft, within the 1-2 km layer above the surface (and just above the top of the moist marine boundary layer, where the stratocumulus cloud field existed). It therefore appears likely that this series of southeastward-moving gravity waves was mildly perturbing the tops of the stratocumulus clouds.

NUCAPS sounding profile for a point over the Gulf of Mexico, north of the Yucatan Peninsula [click to enlarge]

Since there were no nearby surface frontal boundaries or areas of organized deep convection inland over the southern US during the preceding 24 hours, it is unclear as to what may have been the catalyst for these gravity waves.

Fires (as seen on 07 and 10 November) continued to burn in parts of the southeast US on 14 November 2016. A sequence of 3 consecutive true-color Red/Green/Blue (RGB) images from Terra MODIS (1650 UTC), Aqua MODIS (1829 UTC) and Suomi NPP VIIRS (1913 UTC) viewed using RealEarth, above, showed the aerial extent of the dense smoke that was most concentrated over Tennessee, Georgia, North Carolina and South Carolina. With the aid of some of the 16 spectral bands on the ABI instrument aboard GOES-R, true-color images like these will be available at least once every 5 minutes over the Lower 48 states and adjacent areas.

GOES-13 (GOES-East) Visible (0.63 µm) images with plots of surface weather and visibility (below; also available as an MP4 animation) revealed that visibility was restricted to 3 miles or less at one or more sites in all of the aforementioned states. A pair of pilot reports in eastern Tennessee indicated that he top of the smoke layer was at 6000 feet above ground level.

A toggle between Suomi NPP VIIRS Shortwave Infrared (3.74 um) and Day/Night Band (0.7 um) images (with and without METAR surface reports) at 0735 UTC or 3:35 am local time on 15 November(above) showed the “hot spot” signatures and bright glow from the larger fires that were burning in northern Georgia and western North Carolina. With ample illumination from the Moon — which was in the Waning Gibbous phase, at 99% of Full — smoke plumes from some of these fires could be seen drifting southward or southeastward, thanks to the “visible image at night” capability of the Day/Night Band.

During the subsequent daytime hours, Terra MODIS and Suomi NPP VIIRS true-color RGB images (below) again revealed the vast coverage of the thick smoke — and VIIRS Aerosol Optical Depth values were quite high over South Carolina. Unhealthy AQI values persisted during much of the day across parts of Tennessee, Georgia and South Carolina.